Method and apparatus for extending operating range of bar code scanner

ABSTRACT

The operating depth of field for a bar code scanner, preferably a laser scanner, is increased by placing a cubic phase mask (CPM) in the scanning beam. The masked beam is then scanned and reflected off a bar code and received by a photodetector. The received signal is then processed to recover the original unperturbed representation of the bar code pattern. The processed signal has an increased depth of field over an unmasked scanner signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.09/113,523 filed Jul. 10, 1998 now U.S. Pat. No. 6,097,856 and entitledAPPARATUS AND METHOD FOR REDUCING IMAGING ERRORS IN IMAGING SYSTEMSHAVING AN EXTENDED DEPTH OF FIELD, incorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to the field of bar code scanners, and inparticular to extending the depth of field for a laser bar code scanner.

BACKGROUND OF THE INVENTION

Many industries, including the assembly processing, grocery and foodprocessing industries, utilize an identification system in which theproducts are marked with a bar code symbol consisting of a series oflines and spaces of varying widths, or other types of symbols consistingof series of contrasting markings. A number of different bar codereaders and laser scanning systems have been developed to decode thesymbol pattern to a multiple digit representation for inventory,production tracking, and for check out or sales purposes. Opticalscanners are available in a variety of configurations, some of which arebuilt into a fixed scanning station and others of which are portable.The portability of an optical scanner provides a number of advantages,including the ability to inventory products on shelves and to trackportable items such as files, documents, or small equipment. A number ofportable scanners (usually moving beam) use lasers which permit the userto scan the bar code symbols at variable distances from the surface onwhich the bar code is imprinted. However, the possible distances arelimited by the depth of field (DOF).

Various optical readers and optical scanning systems have been developedfor reading bar code symbols appearing on a label or on the surface ofan article. The bar code symbol itself is a coded pattern of indiciacomposed of a series of bars of various widths spaced apart from oneanother to form boundary spaces of various widths, with the bars andspaces having different light reflecting characteristics. The readersand scanning systems transform the graphic indicia information patternsinto electrical signals, which are decoded into alphanumericalcharacters with information content. Such characters are typicallyrepresented in digital form and used as input to a data processingsystem for applications in point-of-sale processing, inventory control,etc.

Bar code symbols are formed from bars or elements that are typicallyrectangular in shape with a variety of possible widths. The specificarrangement of elements defines the character represented according to aset of rules and definitions specified by the symbology used. Therelative width of the bars and spaces is determined by the type ofsymbology used, and the actual size of the bars and spaces is usuallydetermined by the application. The number of characters per inchrepresented by the bar code symbol is referred to as the density of thesymbol. To encode a desired sequence of characters, groups of elementsare concatenated together to form the complete bar code symbol, witheach character of the message being represented by its own correspondinggroup of elements. In some symbologies a unique “start” and “stop”character is used to indicate where the bar code pattern begins andends. A number of different bar code symbologies exist. Thesesymbologies include, e.g., PD417, UPC/EAN, Code 39, Code 49, Code 128,Codabar, and Interleaved 2 of 5, etc.

One embodiment of such a scanning system resides in a hand-held,portable laser scanning head supported by a user which is configured toallow the user to aim the light beam at a symbol to be read. The lightsource in a laser scanner is typically a gas laser or semiconductorlaser. The use of semiconductor devices such as a laser diode as thelight source in scanning systems is especially desirable because oftheir small size, low cost and low power requirements. The laser beam isoptically modified, typically by a condenser lens, to form awaist-shaped beam in which the width of the beam diminishes withdistance until reaching a minimum, or waist, and then increases. Thebeam is focused so that a desired spot size is achieved at the target(bar code) distance, typically so that the waist is located at thetarget distance.

The spot size at the target distance should be approximately the same asthe minimum width between regions of different light reflectivity, i.e.,the width of the bars and spaces of the symbol.

In the scanning systems known in the art, the light beam is directed bya lens or similar optical components along a light path toward a targetthat includes a bar code symbol on the surface. The scanner functions byrepetitively scanning the light beam in a line or series of lines acrossthe symbol. The scanning component may either sweep the beam spot acrossthe symbol and trace a scan line across and past the symbol, or scan thefield of view of the scanner, or do both. Scanning systems also includea sensor or photodetector which functions to detect light reflected fromthe symbol. The photo-detector is positioned in the scanner or in anoptical path in which it has a field of view which extends across andslightly past the symbol. A portion of the reflected light which isreflected off the symbol is detected and converted into an electricalsignal. Electronic circuitry or software decodes the electrical signalinto a digital representation of the data represented by the symbol thathas been scanned. For example, the analog electrical signal from thephotodetector may be converted into a pulse width modulated digitalsignal, with the time intervals proportional to the physical widths ofthe bars and spaces. Such a signal is then decoded according to thespecific symbology into a binary representation of the data encoded inthe symbol.

Bar code symbols are printed in varying densities. High density symbols(bar code element widths <0.007 inch) are, for example, used for smallparts (e.g., integrated circuits) and for symbols with high informationdensity. Low density symbols (bar code element widths >0.020 inch) are,for example, used for coding packages and containers in warehouses. Asit is generally preferred that the beam scanned across the bar codesymbol have a width comparable to that of the minimum width betweenregions of different light reflectivity (e.g., the minimum width of abar element), different beam widths are needed to read different densitybar codes. Furthermore, bar codes of the same density can be located atvarying distances from the laser scanning head.

Conventional laser scanners have a condenser lens that focuses the laserbeam so that the spot size is correct at the range at which the bar codereader is expected to operate. With such fixed focus systems, there istypically a “dead zone” in front of the scanner in which the spot sizeis too large for proper operation. Also, such scanners must be focusedat the factory by adjusting the condenser lens along the optical axiswhile observing the spot size and then permanently setting the positionof the lens at the position that achieves the desired size. This step isa relatively costly one, adding to the cost of manufacturing the laserscanner.

Various proposals have been made for improvements over these fixed focusimaging scanners. U.S. Pat. No. 4,920,255 shows a bar code readingsystem in which the range of the surface bearing the bar code isdetected using an ultrasonic ranging system, and the detected range isused to prescribe the setting of the optics focusing a laser beam on thebar code (the output signal from the ultrasonic ranging system drives astepper motor in the laser focusing optics). U.S. Pat. No. 4,831,275discloses a variety of means for optically modifying the light reflectedfrom the bar code symbol, to vary the distance at which the symbol is infocus on the photodetector within a bar code reader; the techniquestaught include altering the shape of a lens, moving an aperture in theoptical path, moving a mirror (or a fiber optic cable), and providing anarray of sensors, each effectively focused at a different range. U.S.Pat. No. 4,333,006 discloses the use of a plurality of varying focallength holograms placed on a rotating disk to focus at differingoverlapping distance ranges.

A number of proposals have been made to improve the operating depth offield for laser scanners. U.S. Pat. No. 5,723,851 describes a laserscanner incorporating multiple lasers focused for different operatingranges. U.S. Pat. No. 5,302,812 shows a laser scanning head in which therange of the beam waist is varied by moving a condenser lens. U.S. Pat.No. 4,808,804 discloses a number of systems for changing the workingdistance and/or the beam spot size of a laser beam by thelight-transmissive properties of pupils or a movable laser light source.

Obtaining images that are free of errors and distortions introduced bythe optical elements that are used in the imaging process has long beena goal of those working with imaging systems. Such systems contemplatethe imaging of various kinds of objects, including but not limited tobar code symbols, alphanumeric and non-alphanumeric characters andsymbols, and blocks of text. For convenience, all such objects arereferred to herein as target objects, symbols, or indicia, whether theyinclude encoded data or not. The errors and distortions introduced bythe imaging system include, among others, lens aberrations, such asspherical and chromatic aberrations, misfocus errors resulting from anobject being located away from the position of best focus, diffractioneffects produced by aperture stops, and the diffusion effect associatedwith some indicia substrates.

An approach to reducing the magnitude of imaging errors is discussed in“Improvement in the OTF of a Defocussed Optical System Through the Useof Shaded Apertures”, by M. Mino and Y. Okano, Applied Optics, Vol. 10No. 10, October 1971. This article discusses decreasing the amplitudetransmittance gradually from the center of a pupil towards its rim toproduce a slightly better image. “High Focal Depth By Apodization andDigital Restoration” by J. Ojeda-Castaneda et al, Applied Optics, Vol.27 No. 12, June 1988, discusses the use of an iterative digitalrestoration algorithm to improve the optical transfer function of apreviously apodized optical system. “Zone Plate for Arbitrarily HighFocal Depth” by J. Ojeda-Castaneda et al, SPIE Vol. 1319 Optics inComplex systems (1990) discusses use of a zone plate as an apodizer toincrease focal depth. While all of these approaches achieve someimprovement in image quality, they all have features that limit theirusefulness in particular applications, such as bar code reading.

Another approach to reducing the magnitude of misfocus errors is toinclude appropriate phase masks in the imaging system. One example ofthis approach is described in U.S. Patent No. 5,748,371 (Cathey et al.).In this patent, the imaging system comprises a lens or lenses and anopto-electronic image sensor. It also includes a cubic phase mask (CPM)which is located at one of the principal planes of the imaging system,and which modifies the optical transfer function (OTF) of the imagingsystem in a way that causes it to remain approximately constant oversome range of distances that extends in both directions (i.e., towardsand away from the lens) from the distance of optimum focus. Theintermediate image produced by the image sensor is then digitallypost-processed to recover a final image which has a reduced misfocuserror. While the image correcting technique described above producesresults that are substantially better than the results produced bypurely optical means, our efforts to use phase masks in imaging typeoptical readers using this technique have produced unsatisfactoryresults.

SUMMARY OF THE INVENTION

Briefly stated, the operating depth of field for a bar code scanner,preferably a laser scanner, is increased by placing a cubic phase mask(CPM) in the scanning beam. The masked beam is then scanned andreflected or scattered off a bar code and received by a photodetector.The received signal is then processed to recover the originalunperturbed representation of the bar code pattern. The processed signalhas an increased depth of field over an unmasked scanner signal.

According to an embodiment of the invention, an imaging system forimaging an object located in a target region includes an illuminationsource; an optical path between the illumination source and the object;a first optics assembly in the optical path; phase masking means in theoptical path for receiving light and modifying a phase thereof as afunction of position within the phase masking means, thereby creatingphase modified light having known effects therein; means for traversingthe phase modified light across the object in the target region; imagesensing means for receiving light reflected from the object andproducing an intermediate image signal therefrom; the intermediate imagesignal having at least a misfocus error that is dependent upon adistance between the first optics assembly and the object; andprocessing means for correcting for the known effects of the phasemodified light to produce a final image signal of the object. The finalimage thus has a reduced amount of out of focus error over a range ofdistances.

According to an embodiment of the invention, a method of scanning for alaser scanning system suited for reading indicia located in a targetregion, including the steps of (a) generating an illumination beam forilluminating an indicia located in an operational depth of field; (b)changing locally a phase of the illumination beam as a function ofposition before the beam illuminates the indicia; (c) changing locallyan amplitude of the illumination beam as a function of position beforethe beam illuminates the indicia; (d) receiving light reflected from theindicia; (e) converting the received light to an intermediate imagesignal; and (f) processing the intermediate image signal such that theoperational depth of field is extended.

According to an embodiment of the invention, a method of modifying alaser beam to maximize the system resolving ability as a bar codeindicia is moved throughout an operational depth of field includes thesteps of (a) generating a laser illumination beam; (b) modifying a phaseof the illumination beam a function of position in the beam; (c)modifying an amplitude of the illumination beam as a function ofposition in the beam; (d) scanning the phase and amplitude-modified beamacross the bar code indicia; (e) receiving light reflected from the barcode indicia; (f) converting the received light to an intermediate imagesignal; and (g) processing the intermediate image signal using aprecalculated recovery function to reduce effects of a position of thebar code indicia.

According to an embodiment of the invention, a method of scanning abarcode indicia, the indicia containing information encoded therein,includes the steps of (a) illuminating the indicia in an operating rangewith light; (b) modifying an amplitude and phase of the light to reducevariations in a localized illumination distribution of the light overthe operating range before the light illuminates the indicia; (d)directing the light to the operating range; (e) scanning the lightacross the indicia; (f) receiving light scattered from the indicia; (g)converting the scattered light to an intermediate image signal; and (h)processing the intermediate image signal to recover the informationencoded in the indicia.

According to an embodiment of the invention, a method of increasing anoperating depth of field includes (a) providing illumination; (b)distorting a phase of the illumination in a predetermined manner at aspecific distance from an object; (c) directing the distortedillumination to the object; (d) receiving light scattered from theobject; (e) converting the received light to an intermediate imagesignal; and (f) processing the intermediate image signal using aprecalculated recovery function to reduce effects of the specificdistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a high level block diagram of a prior art imaging systemthat includes a cubic phase mask and a digital signal processor thatuses a known recovery function and a known image recovery algorithm.

FIG. 2 shows a high level block diagram of an imaging system thatincludes an optical assembly constructed in accordance with theinvention and a digital signal processor that uses the generalizedrecovery function and recovery algorithm of the invention.

FIG. 3A shows an exemplary embodiment of a 1D imaging system thatincludes optical assemblies and digital signal processors of the typecontemplated by the invention.

FIG. 3B shows an exemplary embodiment of a 1D imaging systems thatincludes optical assemblies and digital signal processors of the typecontemplated by the invention.

FIG. 3C shows a front view of a 1D amplitude mask suitable for use inthe embodiment of FIG. 3A.

FIG. 3D shows a side view of an optical element in which a focusingelement, a phase mask and an amplitude mask are formed on differentsurfaces thereof.

FIG. 4 shows an opto-electronic block diagram of a 1D optical readerwhich includes an imaging system of the type shown in FIG. 3A or FIG.3B.

FIG. 5A shows an exemplary embodiments of a 2D imaging system thatincludes optical assemblies and digital signal processors of the typecontemplated by the invention.

FIG. 5B shows an exemplary embodiments of a 2D imaging system thatincludes optical assemblies and digital signal processors of the typecontemplated by the invention.

FIG. 6 shows an optical-electronic block diagram of a 2D optical readerwhich includes an imaging system of the type shown in FIG. 5A or FIG.5B.

FIG. 7A shows the calculated in-focus line spread function for a 1Dimaging system having an optical assembly with a sharply definedaperture and a CPM.

FIG. 7B shows the calculated in-focus line spread function for a 1Dimaging system similar to that of FIG. 7A, except that it has an opticalassembly which includes an amplitude mask having a Gaussian transmissionprofile.

FIG. 7C shows the measured in-focus point spread function for a 2Dimaging system having an optical assembly with a sharply definedaperture and a CPM.

FIG. 7D shows the measured in-focus point spread function for a 2Dimaging system similar to that of FIG. 7C, except that it has an opticalassembly which includes an amplitude mask having a Gaussian transmissionprofile.

FIG. 8 illustrates the steps used to produce exemplary forms of therecovery function of the invention.

FIG. 9A illustrates the steps used in applying the recovery function ofthe invention in the spatial frequency domain.

FIG. 9B illustrates the steps used in applying the recovery function ofthe invention in the spatial domain.

FIG. 10 includes selected equations that illustrate terms and functionsthat are used in producing the recovery function of the invention.

FIG. 11 shows the calculated intermediate image signal which areproduced when the 1D imaging system of the invention is used with therecovery function, but without the amplitude mask of the invention.

FIG. 12 shows the calculated final image signal which are produced whenthe 1D imaging system of the invention is used with the recoveryfunction, but without the amplitude mask of the invention, as obtainedfrom the intermediate signal of FIG. 11.

FIG. 13 shows how the intermediate image signals of FIG. 11 is changedby the addition of an amplitude mask.

FIG. 14 shows how the final image signals of FIG. 12 is changed by theaddition of an amplitude mask.

FIG. 15 shows a bar code laser scanning system according to anembodiment of the present invention.

FIG. 16A shows a high level block diagram of an imaging system thatincludes an optical assembly constructed in accordance with theinvention and a digital signal processor that uses the generalizedrecovery function and recovery algorithm of the invention.

FIG. 16B shows a an opto-electronic block diagram of a 1D optical readerwhich includes an illumination system of the type shown in FIG. 15 orFIG. 16A.

FIG. 17 shows a specific example of a 1-D laser scanning opticalassembly that includes an amplitude mask and a phase mask that issuitable for practicing an embodiment of the invention.

FIG. 18 shows a bar code laser telocentric scanning system according toan embodiment of the present invention.

FIG. 19A shows a scan profile made with a conventional laser scanner ata target distance of 410.0 mm.

FIG. 19B shows a scan profile made with a conventional laser scanner ata target distance of 402.0 mm.

FIG. 19C shows a scan profile made with a conventional laser scanner ata target distance of 391.0 mm.

FIG. 19D shows a scan profile made with a conventional laser scanner ata target distance of 378.0 mm.

FIG. 20A shows a scan profile of the intermediate signal made with thepresent invention at a target distance of 351.5 mm.

FIG. 20B shows a scan profile of the reconstructed digital signalcorresponding to the intermediate signal of FIG. 20A made with thepresent invention at a target distance of 351.5 mm.

FIG. 21A shows a scan profile of the intermediate signal made with thepresent invention at a target distance of 409.0 mm.

FIG. 21B shows a scan profile of the reconstructed digital signalcorresponding to the intermediate signal of FIG. 21A made with thepresent invention at a target distance of 409.0 mm.

FIG. 22A shows a scan profile of the intermediate signal made with thepresent invention at a target distance of 420.0 mm.

FIG. 22B shows a scan profile of the reconstructed digital signalcorresponding to the intermediate signal of FIG. 22A made with thepresent invention at a target distance of 420.0 mm.

FIG. 23A shows a scan profile of the intermediate signal made with thepresent invention at a target distance of 457.0 mm.

FIG. 23B shows a scan profile of the reconstructed digital signalcorresponding to the intermediate signal of FIG. 20A made with thepresent invention at a target distance of 457.0 mm.

FIG. 24 shows a method of implementing the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Original material from parent application.

Referring to FIG. 1 there is shown a prior art imaging system which isconfigured to produce a digital representation of an object 10. Thisimaging system includes an optical assembly 12 having a lens or set oflenses 14 and a phase mask 16 which is preferably, although notnecessarily, of the cubic type or at least of the type which can bespecified in terms of a third order polynomial. The exit pupil ofoptical assembly 12 may be defined either by an aperture stop (notshown) or by the lateral dimensions of one of its lenses, as indicatedgenerally by the dotted line labeled 18. The imaging assembly of FIG. 1also includes an opto-electronic image sensor 30 for receiving the realintermediate image of object 10 formed thereon by optical assembly 12,and converting that light image into an intermediate image signal which,after undergoing an analog to digital conversion, becomes a digitalintermediate image signal that is made up of a number of discrete imagedata elements. Finally, the imaging system of FIG. 1 includes a digitalsignal processor (herein often abbreviated to DSP) that stores andpost-processes the image data elements of the intermediate image signalto produce a set of final image data elements which has been correctedin the manner described in U.S. Pat. No. 5,748,371, which is herebyincorporated herein by reference.

As explained in the above-mentioned patent, phase mask 16 is placed at aprincipal plane of lens 14, and causes the OTF of the imaging system toremain approximately constant over some range of distances from thein-focus position. DSP 35 stores and post-processes the intermediateimage signal produced by image sensor 30 to produce a corrected digitalrecovered (final) image signal which has a reduced amount of out offocus error over a range of distances. This is stated to be accomplishedby implementing a recovery function that is based upon an approximatedsystem OTF.

While post-processing of the intermediate image signal of the imagingsystem of FIG. 1 in this manner does reduce the out of focus errorthereof, the magnitude of this reduction is suboptimal. In particular,the system OTF approximation fails to take into account an abrupt changein phase at the origin of the incoherent OTF (as defined in the abovecited patent) that is produced by the cubic phase mask. This, in turn,causes the final image produced by DSP 35 to have substantialdistortion.

In addition to including the above-mentioned residual out of focuserror, the recovered images produced by the imaging system of FIG. 1will include the uncompensated errors introduced by the elements of itsoptical assembly. These errors may include coma associated with lens 14and diffraction errors produced by aperture stop 18, among others.Together with the partly corrected out of focus error, these errors canresult in recovered images which are too distorted to be useful indevices, such as optical readers, which operate best when they arepresented with images that have sharply defined black-white andwhite-black transitions.

During the making of the invention, it was discovered that there existsa deficiency in the above-discussed method for reducing out of focuserror, and that this deficiency can be corrected in a way thatsimultaneously corrects for a variety of other imaging system errors. Itwas also discovered that this more powerful and more inclusive type ofcorrection can be accomplished without significantly increasing theamount of image processing that must be done. As explained more fullylater, the method of the invention in part involves the making ofimprovements to the optical assembly of the imaging system, and in partinvolves the making of improvements to the techniques used inpost-processing the image signal produced thereby.

In FIG. 2 there is shown the overall configuration of an imaging systemconstructed in accordance with the method and apparatus of theinvention. The imaging system of FIG. 2 includes an incoherentillumination source 21, an optical assembly 20 having a lens or set oflenses 22, a phase mask 24, and an amplitude mask 26 which preferably,although not necessarily, has a Gaussian transmission profile. A numberof exemplary physical embodiments of optical assembly 20 are describedwith reference to FIGS. 3A, 3B, 3C, 5A and 5B. The imaging system ofFIG. 2 also includes an image sensor 30 for receiving the intermediateimage formed by optical assembly 20 and converting that image into anintermediate image signal that, after A/D conversion, becomes a digitalintermediate image signal which is made up of a plurality of discreteimage data elements. Finally, the imaging system of FIG. 2 includes aDSP 40 for storing and post-processing the digital intermediate imagesignal in accordance with a recovery algorithm that uses the improved,generalized recovery function of the invention, this difference beingindicated by the assignment of a new number 40. This recovery algorithmand function are described later with reference to FIGS. 8 and 9A.

Referring to FIG. 3A there is shown a side view of a first embodiment ofa 1D imaging system constructed in accordance with the invention. Thisimaging system includes a 1D optical assembly 20-1, a 1D image sensor30-1, and a DSP 40-1 which is adapted to implement a 1D version of therecovery function of the invention. In the embodiment of FIG. 3A opticalassembly 20-1 includes a lens 22-1, a 1D phase mask 24-1 (the curvatureof which is greatly exaggerated to clearly show its shape), and a standalone 1D amplitude mask 26-1. In the preferred embodiment, 1D amplitudemask 26-1 is rectangular in form. As shown in FIG. 3C, the transmittanceof mask 26-1 decreases smoothly and continuously as a function of thedistance from the center line of the amplitude mask, along the X-axisuntil it reaches or approximates a value of zero at the X boundaries X1and X2 thereof. This center line is located on the optical axis ofoptical assembly 20-1. This optical axis is perpendicular to the planethe object shown in FIG. 3C. The transmittance of mask 26-1 is invariantin directions parallel to the Y axis of assembly 20-1 and, when usedwith a 1D image sensor, approximates an aperture that is unbounded inthat direction. Although apertures having other shapes can be used, theyresult in recovery algorithms or functions that are significantly morecomplex than those for a rectangular aperture and, consequently, are notpreferred.

Phase mask 24-1 is located at or near the front principal plane of lens22-1 and preferably, although not necessarily, has a phasecharacteristic which is cubic in form, i.e., which can be specified by athird order polynomial. During imaging, phase mask 24-1 operates in aknown manner to cause the OTF of optical assembly 20-1 to remainapproximately constant over a range of distances that extends in bothaxial directions from the location of optimum focus (i.e. towards andaway from the lens). In the preferred embodiment, the coefficients ofthe cubic or third order polynomial function are selected according tothe maximum amount of misfocus that the imaging system is designed toaccommodate, and are just sufficient to compensate for that degree ofmisfocus. Even though the phase mask technique can compensate certainother types of errors, such as spherical aberration, it is contemplatedin the invention that all such known errors are compensated, ifpossible, in the recovery function, and the phase mask technique will beused to correct only misfocus error, which is not known a priori. Thisassures that the size of the PSF that results from the presence of thephase mask is no larger than is actually required, and does notunnecessarily decrease the signal to noise ratio of the digitalintermediate image signal. The magnitude of the cubic phase functionparameter is related to the size of the aperture of the optical assemblyand the cubic phase coefficient in the manner indicated by Equation 8 ofFIG. 10.

The presence of amplitude mask 26-1 represents a departure from andimprovement over imaging systems that have previously been used inindicia readers. This amplitude mask defines for the imaging system as awhole a “soft aperture” which reduces the magnitude of the diffractionripples that are associated with the intermediate PSF in opticalassemblies that have “hard apertures” i.e., aperture stops with sharplydefined edges. This amplitude mask has the effect of smoothing out theintermediate image PSF. This smoothing may be seen by comparing FIG. 7A,which shows the in-focus intermediate image PSF of a 1D optical assemblywhich has a “hard aperture”, and FIG. 7B, which shows the intermediateimage PSF of an optical assembly such as that shown in FIG. 3A whichincludes a 1D Gaussian amplitude mask. Amplitude mask 26-1 that has aGaussian characteristic provides the additional advantage that it causesthe intermediate image PSF to have an envelope shape that does notchange as much over a depth of focus as that of an optical systemimplemented without an amplitude mask. A mathematical function whichdescribes a 1D amplitude mask that has Gaussian characteristics is shownin Equation 4 of FIG. 10.

While FIG. 3A shows amplitude mask 26-1 as a stand alone mask, it is notessential that it be a stand alone element. Mask 26-1 may, for example,be formed on or as a part of phase mask 24-1 or lens 22-1. Mask 26-1 mayalso be formed as a part of a single, composite element that combinesthe functions of all of lens 22-1, phase mask 24-1 and amplitude mask26-1. A single element of this type is shown in FIG. 3D. In the latterfigure, a single, composite optical element 27 includes a front surface22-A which serves as a focusing structure, a rear surface 24-A whichserves as a phase mask, and a coating 26-A which serves as an amplitudemask.

If the transmittance of the mask is made to become equal to zero beforeanother optical element, such as a lens, limits the aperture of theoptical assembly, then the locus of points at which the transmittancebecomes equal to zero defines the aperture stop of the optical assembly,without regard to where the physical edges of the mask are. If thetransmittance of the mask has a non-zero value at the edges of the mask,and no other optical element defines a smaller aperture stop for theoptical assembly, the edges of the mask define the aperture stop of theoptical assembly. In the latter case, the fact that the transmittance isnot equal to zero at the edge of the mask does not prevent the mask fromestablishing the desired soft aperture, provided that the transmittanceis small enough to not be of practical significance.

In addition, mask 26-1 may define an amplitude mask having anon-Gaussian transmission profile, provided that the change in itstransmittance has a gradient or curvature that decreases smoothly acrossthe width of the mask. Examples of non-Gaussian functions that may besuitable for use in establishing such smoothly decreasing gradientsinclude exponential and Bessel functions, and segments of cosinefunctions.

Sensor 30-1 may be any of a number of different types of opto-electronicimage sensors, including CCD, CID and CMOS sensors, among others. Sensor30-1 should have enough photosensitive elements to provide a spatialsampling resolution sufficient to meet the requirements of theapplication in which the imaging system is used. For many 1D bar codereading applications, an image sensor having an imaging array with 600or more photosensitive elements is sufficient. The image data producedby these photosensitive elements taken together comprise an intermediateimage signal that corresponds to the real intermediate image that wasformed on the imaging array of the image sensor. In order for thisintermediate image signal to be post-processed in accordance with theinvention, it must first be stored in memory. As a result, theintermediate image signal must have a digital , e.g., gray scale,format. Accordingly, if the image sensor is not one that makes itsoutput available as a digital intermediate image signal, it should beconverted to such a signal by a suitable A/D converter.

While the intermediate image appearing at the active surface of imagesensor 30-1 is a continuous image, the digital intermediate image signalis a discontinuous image that is made up of a plurality of discreteimage data elements. Each of these elements has an amplitude which isproportional to the time averaged result of the superposition of thesquares of the absolute values of the coherent PSFs of all points of theobject that are projected onto the respective photo sensitive elements.As explained more fully later, it is the function of DSP 40-1 to recovera representation of the original object which has been compensated formisfocus and for as many of the errors introduced by the associatedoptical assembly as is practicable. This operation is referred to hereinas the “recovery process” or “recovery algorithm”, and makes use of thegeneralized recovery function of the invention.

Referring to FIG. 3B there is shown a second embodiment of a 1D imagingsystem constructed in accordance with the invention. The embodiment ofFIG. 3B differs from that of FIG. 3A primarily in that its phase mask24-2 is located at the rear rather than the front principal plane of itslens 22-2. In addition, the amplitude mask 26-2 of the imaging system ofFIG. 3B takes the form of a film deposited on the rear surface of phasemask 24-2. Because the statements made in connection with the embodimentof FIG. 3A are equally applicable to the embodiment of FIG. 3B, theembodiment of FIG. 3B will not be discussed in detail herein.

As appreciated by those skilled in the art, it is the 1D character ofimage sensor 30-1 or 30-2 that is responsible for the fact that theimaging systems of FIGS. 3A and 3B are termed 1D imaging systems. As aresult, in spite of the fact that the imaging systems of FIGS. 3A and 3Bare 1D imaging systems, optical assemblies 20-1 and 20-2 may beconstructed using either 1D or 2D versions of their phase and amplitudemasks or any convenient combination thereof. Lenses 22-1 and 22-2, onthe other hand, are preferably 2D lenses. It is understood that all suchversions and their equivalents are within the contemplation of thepresent invention.

Referring to FIG. 4 there is shown a block diagram of an apparatus inwhich the imaging systems of FIGS. 3A and 3B can be used, namely: a 1Dhand held bar code reader. In addition to optical assembly 20 and imagesensor 30 this reader includes an illumination system 50 which includesa linear array of LEDs 52 and a focusing mirror 54 together with asuitable LED driver circuit 56. Most of the remainder of the reader ofFIG. 4 comprises the circuit elements that have thus far been referredto collectively as DSP 40. These circuit elements include an analogsignal processing circuit 32 and an analog to digital converter 34 forconverting the analog output signal of image sensor 30 into anintermediate image signal that can be processed using digital methods.These circuit elements also include microcomputer 42, a read only memory(ROM) 44, a read write memory (RAM) 46, and a bus 48 for interconnectingthem. A suitable DMA controller 43 may also be included in order tohandle the storage of image data, thereby allowing computer 42 toconcentrate on the tasks of post-processing the digital intermediateimage signal to recover and decode the final image data. The operationof the reader of FIG. 4 will typically be initiated either manually, bymeans of a suitable trigger T, or non-manually by means of anautotrigger arrangement.

The overall operation of the reader of FIG. 4 is controlled by a programstored in ROM 44. Generally speaking, this program includes a readercontrol portion, a decoding portion, and an image recovery portion. Thereader control portion of the program deals with matters such as thestarting and stopping of scanning, and the inputting and outputting ofdata via an I/O interface 49. The decoding portion deals with matterssuch as identifying the symbology used in the symbol being read and thedecoding of that symbol in accordance with the rules that govern thatsymbology. Because programs for performing these functions are includedin commercially available readers, such as the model numbers ST 3400 andIT 4400 readers sold by the assignee of the present invention, they willnot be discussed in detail herein. The image recovery portion of theprogram is discussed later in connection with FIGS. 9A and 9B.

Referring to FIGS. 5A and 5B there are shown two embodiments of 2Dimaging systems that are constructed in accordance with the invention.As in the case of the 1D embodiments of FIGS. 3A and 3B, the 2Dembodiments of FIGS. 5A and 5B each include an optical assembly, animage sensor, and a DSP, each of which is labeled with the same numberused in earlier discussed embodiments, except for changes in postscript.In the embodiment of FIG. 5A, the lens 22-3 is a 2D lens similar to thatused in the 1D embodiments of FIGS. 3A and 3B. The phase and amplitudemasks of FIG. 5A are similar to their counterparts in FIG. 3A, exceptthat they are preferably square and provide for the additional dimensionin a rectilinear manner. The amplitude mask of FIG. 5A is made up of twoidentical 1D amplitude masks 26-3A and 26-3B which are oriented at rightangles to one another. With this angular orientation, the two 1Damplitude masks together function as a 2D amplitude mask. The 2Damplitude mask may also be made as a single element. Similarly, thephase mask of FIG. 5A is made up of two identical 1D phase masks 24-3Aand 24-3B which are oriented at right angles to one another. With thisangular orientation, the two 1D phase masks together function as asingle 2D phase mask to facilitate the compensation of misfocus error in2D images. The 2D phase mask may also be made as a single element 24-4,as shown in the embodiment of FIG. 5B. Since the other statements madeearlier with reference to the optical assemblies of FIGS. 3A and 3B areequally applicable to the optical assemblies of FIGS. 5A and 5B, exceptfor the effect of the added dimension, the optical assemblies of FIGS.5A and 5B will not be further discussed herein.

Unlike the image sensors shown in FIGS. 3A and 3B, the image sensorsshown in FIGS. 5A and 5B are 2D image sensors and, consequently, willhave hundreds of times the number of photosensitive elements as their 1Dcounterparts. In addition, the DSPs of FIGS. 5A and 5B, although notsubstantially different from their counterparts in FIGS. 3A and 3B froman electrical standpoint, are arranged to use recovery algorithms whichcan apply either 1D or 2D recovery functions, depending on how they areused. If, for example, a 2D image sensor images a 2D matrix bar codesymbol, the reader should apply a 2D recovery function. If, on the otherhand, a 2D image sensor images a 1D bar code symbol two-dimensionally,it should also be able to take a 1 D slice of the stored representationof that symbol and process that slice in accordance with a 1D recoveryfunction. It will therefore be understood that there is no inherentrelationship between the number of dimensions that characterize therecovery functions of the invention and the number of dimensions thatcharacterize the image sensors with which these functions are used.

Although 2D recovery functions are more complex than their 1Dcounterparts, they are equally familiar to those skilled in the art.Accordingly, in order to avoid needless repetition, the presentdescription will frame its discussion of the recovery algorithm and itsrecovery function in terms of a 1D intermediate image signal, and leaveit to those skilled in the art to adapt the description as necessary toprocess a 2D intermediate image signal.

A specific example of a 2D optical assembly that includes an amplitudemask that is suitable for practicing an embodiment of the presentinvention is shown in Table 1 below.

TABLE 1 (Reference the equations of FIG. 10.) C03 = 0.0787 (1/inch)²cubic coefficient for CPM n = 1.489 refractive index of CPM (acrylic) f= 7.93 inch focal length of biconvex lens λ = 660 nm wavelength of lightp = q = 15.859 inch image and object distances element separation = 7.4μm SONY ICX084AL 2D imager element separation imager array = 659 × 494elements imager array format σ = 0.239 inch Gaussian coefficient

Referring to FIG. 6, there is shown an opto-electronic block diagram ofone exemplary 2D bar code reader that is suitable for use with thepresent invention. In addition to a 2D optical assembly and a 2D imagesensor, such as 20-4 and 30-4 of FIG. 5B, this reader includes anillumination system 50-4 which includes a 2D array of LEDs 52-4 togetherwith a suitable LED driver circuit 56-4. Most of the remainder of FIG. 6comprises the circuit elements which have thus far been referred to asDSP 40-3 or 40-4. These circuit elements include an analog signalprocessing circuit 32-4 and an A/D converter 34-4 for converting theoutput signal of image sensor 30-4 into a digitally processablerepresentation of the 2D intermediate image of object 10-4. Thesecircuit elements also include microcomputer 42-4, a ROM 44-4, a RAM 46-4and a bus 48-4 for interconnecting them. A suitable DMA controller 43-4may also be included to handle the storing of 2D image data, therebyallowing computer 42-4 to concentrate on the tasks of post-processingthe image data to recover and decode the final image. Since a DMAcontroller of this type is used in known, commercially available barcode readers, such as in the earlier mentioned model IT 4400 reader,this controller will not be further described herein.

Because a 2D image includes thousands of image data elements, andbecause these data elements will ordinarily be processed using asuitable transform pair, such as the Discrete Fast Fourier Transform(DFFT) and Inverse Discrete Fast Fourier Transform (IDFFT), the DSP ofthe invention may also include a dedicated circuit, which may take theform of an ASIC, for performing these transforms. Equivalently, the DSPmay be provided with a second, general purpose DSP (not shown) which isdedicated to executing the routines which implement the DFFT-IDFFTtransform pair, and making the results available to main DSP 42-4. Ifsuch a second DSP is used, other tasks, such as the application of therecovery function and the execution of decoding programs may be dividedup in various ways between them. It will therefore be understood thatthe present invention is not limited to the use of any particular numberof microcomputers and/or DSPs or to any particular allocation of tasksamong them, and that all such numbers and allocations are within thecontemplation of the invention if they use the image recovery techniquesdiscussed herein or their equivalents.

The overall operation of the reader of FIG. 6 is controlled by a programstored in ROM 44-4. Generally speaking, this program includes a readercontrol portion, a decoding portion and an image recovery portion. As inthe case of the 1D reader of FIG. 4, the reader control and decodingportions of 2D reader of FIG. 6 are of types known to those skilled inthe art and is not discussed in detail herein. The image recoveryfunction, and the overall image recovery process of which the recoveryfunction forms a part, is now described with reference to FIGS. 8through 10.

Referring to FIG. 8, there is shown a high level representation of oneexemplary sequence of steps that may be used to produce the preferredembodiment of the generalized recovery function of the invention. Thefirst of these steps, 8-1, includes the determination of the calculatedgeneralized pupil function of the optical assembly as it exists under acondition of approximately optimum focus, with the phase mask in place,a quantity which is referred to herein as H(W/PM). This pupil functionis equal to the product of a plurality of terms, each of which comprisesa mathematical function that specifies a respective one of thecharacteristics of the elements of optical assembly 20. Principal amongthese functions are: an aperture function which mathematically describesof the amplitude mask 26 and the effect of other optical elements, ifany which act as an aperture stop, a phase mask function thatmathematically describes phase mask 24, and a lens function 22 thatmathematically describes the lens. In the event that the amplitude maskalone defines the aperture stop of the optical assembly, as may be thecase when the transmittance of the mask is made to equal zero before itsphysical boundaries are reached, the aperture function becomes the sameas the function which mathematically describes the amplitude mask, andmay properly be referred to as an amplitude mask function. This pupilfunction may also include other terms (other than an out of focus term)that mathematically describe effects which are able to introduce errorsinto the image which the optical assembly produces on image sensor 30,such as spherical aberrations or astigmatism in the entire opticalsystem (such as the window). An out of focus term is not includedbecause, as stated earlier, the calculated pupil function is calculatedunder a condition of approximately optimum focus. The specific focusconditions are not known a priori and therefore can not be put into ageneralized recovery function. The recovery function is calculated fromthe calculated pupil function. Since the recovery function can reducethe error associated with an effect only if that effect is taken intoaccount in the calculated pupil function, it is desirable to include inthe calculated pupil function terms that represent as many errorproducing effects as is practicable.

The mathematical expression which describes the pupil function dependson the actual sizes, shapes and locations of each physical element thatforms a part of the optical assembly, and cannot therefore be writtenout in exact numerical form independently of the optical assembly towhich it relates. It can, however, be written out in symbolic form, andwhen so written out, has the general form and includes terms such asthose shown in Equation 1 in FIG. 10. Examples of equations that showthe mathematical forms of the terms that are included in Equation 1 areincluded as Equations 4 through 6 of FIG. 10. Of these, Equation 4 showsa Gaussian amplitude mask function, Equation 5 shows a cubic phase maskfunction, Equations 6 and 7 together show aberration functions. Furtherinformation concerning the last mentioned function may be found on page2-19 of Handbook of Optics, edited by W. G. Driscoll and W. Vaughan,McGraw-Hill, 1978. This formulation assumes that all of the physicalobjects in the optical assembly can be considered to be located in theplane of the exit pupil. If this is not the case, then the mathematicalfunctions describing the individual objects can be mapped or referred tothe plane of the exit pupil. Those skilled in the art will understandwhy this is desirable and how it can be accomplished.

After the approximately in-focus generalized pupil function has beendetermined, the next step in producing the recovery function of theinvention is to produce a mathematical representation of the incoherentoptical transfer function (IOTF) of the optical assembly atapproximately best focus, with the phase mask in place, a quantity whichis referred to herein as IOTF(W/PM). This representation may begenerated in either of two ways. One is to calculate the autocorrelationfunction of H(W/PM) directly, using equations known to those skilled inthe art as, for example, are described on page 139 of the above-citedbook by Goodman. An equation of this type is included herein as Equation2 of FIG. 10. Although this calculation is time consuming, its use ispractical because it need be done only once, during the design phase ofa new imaging system, rather than each time a new image is to beprocessed. The second, preferred way of producing the IOTF includescarrying out steps 8-2 through 8-4 of FIG. 8. These include taking theIDFFT of H(W/PM), squaring its absolute value, and taking the DFFT ofthat squared absolute value. These and all substantially equivalentmethods for generating the IOTF(W/PM) of the optical assembly isunderstood to be within the contemplation of the invention.

In the spatial frequency domain, the inverse (in the sense ofreciprocal) of the IOTF(W/PM) may be used by itself as the recoveryfunction, albeit in its simplest, least practical form. Although thisform may be viable in principle, it is not practical for use in realsystems because of the impact of noise, truncation errors, and othercalculation errors. A more practical form of the recovery function canbe produced, however, if IOTF(W/PM) is used as the denominator of afraction, the numerator of which is the IOTF without the phase mask, orother aberrations at approximately optimum focus, namely IOTF(W/OPM). Asin the case of the IOTF(W/PM), the IOTF(W/OPM) may be produced bycalculating the ideal generalized pupil function without the phase mask,error or aberrations, but with the amplitude mask of the opticalassembly, H(W/OPM), as shown in step 8-5 of FIG. 8, and then eithertaking the autocorrelation of H(W/OPM) or performing the steps 8-6through 8-8 of FIG. 8. When the result, IOTF(W/OPM), is divided byIOTF(W/PM), as shown in FIG. 8, the resulting fraction comprises thebasic recovery function of the present invention. The use of thisrecovery function is discussed presently in connection with FIGS. 9A and9B.

When calculating the numerator and/or denominator, it is understood thatfor a specific practical application one may want to optimize the systemabout a position other than the position of optimum focus. It isunderstood that the use of the non-optimum focus condition for thegeneration of a recovery function is also within the scope of thisinvention.

The above-discussed, basic recovery function of the invention may befurther improved by multiplying it by a further filtering, waveshapingor finishing function, hereinafter referred to as FILTER, which servesto optimize the basic recovery function by altering the shape thereof,and thereby giving it a more “finished” form. This filtering function isa function which may be arrived at either empirically or by calculation,and which takes into account the problems associated with particularapplications. These may include problems such as image contrast, thediffusion effect, quantization error, off-axis error and the effect ofsparse sampling. The mathematical form of an exemplary filter functionof this type is shown in Equation 3 of FIG. 10.

Referring to FIG. 9A, there is shown an exemplary embodiment of a methodfor using the recovery function of the invention, in this case a methodfor using it to read a bar code symbol. In FIG. 9A, the first step 9-1includes the step of acquiring the image to be read. In the 2D reader ofFIG. 6, this step includes the illumination of the target symbol, theformation of an intermediate image having an unknown degree of misfocuson image sensor 30-4, and the conversion of that image into anintermediate image signal (IIS). In step 9-2, this IIS is converted to adigital IIS (if necessary) by A/D converter 34-4 and stored in RAM 46-4,preferably as an array or matrix of gray scale image data values, andthereby made available for post-processing. In the embodiment of FIG.9A, this post-processing includes the use of the recovery function inthe spatial frequency domain and the use of an IDFFT to return theresults to the spatial domain for final bar code decoding. Thispost-processing may also be performed in the spatial domain, asexplained later in connection with the embodiment of FIG. 9B.

In step 9-3 of FIG. 9A, the recovery function, preferably with afiltering function appropriate for the application, is retrieved fromROM 44-4, where it is stored at the time of manufacture, and madeavailable to the processor which will use it (step 9-3). A DFFT is thenperformed on the stored digital IIS (step 9-4) and the resultingspectrum of the digital IIS, which takes the form of an array or matrixof complex numbers, is multiplied by the recovery function (step 9-5),on an element by element basis, to produce a recovered spectrum of thedigital IIS. In accordance with the invention, the latter spectrumcomprises a spatial frequency domain representation of the read imagewhich is compensated for the unknown degree of misfocus and othersources of error that were taken into account in the recovery function .Once this recovered spectrum is available, an IDFFT is performed thereonto produce a recovered or final image signal (step 9-6). Once thisrecovered image signal is stored in memory, it is ready for decoding ina manner known to those skilled in the art (step 9-7).

FIG. 9B illustrates how the method and apparatus of the invention may bepracticed in the spatial domain. The first two steps, 9-1 and 9-2, theacquisition, conversion and storage of an intermediate image are thesame as those discussed in connection with FIG. 9A. The next two steps,9-10 and 9-11, may include the accessing of the recovery function of theinvention, as defined in FIG. 8, and the taking of the IDFFT thereof toproduce a spatial domain representation of that recovery function.Alternatively, if the IDFFT is generated and stored in the reader at thetime of manufacture, the latter steps may be combined into a single step9-12, which includes the retrieval of the spatial domain representationof the recovery function.

Once the spatial domain representations of the digital IIS and therecovery function are both available, the latter is applied to theformer in step 9-14 to produce a spatial domain recovered image signalby convolving the recovery function with the digital IIS. If the opticalsystem is not shift or spatially invariant, the convolution integralshould not be used, and the more general superposition integral be usedinstead. Since both of these integrals are well known to those skilledin the art, the particulars of the application thereof are not discussedin detail herein. Similar conditions are understood to apply to thespatial frequency domain representation of the digital IIS, i.e., thespectrum of the digital IIS. More particularly, if the optical system isnot shift or spatially invariant, then the Fresnel approximation to theKirchoff or the Rayleigh-Sommerfeld equations (See page 51, equation3-48 of the earlier mentioned Goodman reference) which permits the useof the Fourier transform method is probably not valid, and fulldiffraction theory must be used instead.

The recovery methods illustrated in FIGS. 9A and 9B are understood to besubstantially equivalent to one another. This is because multiplicationin the spatial frequency domain is the equivalent of convolution in thespatial domain via the convolution theorem for Fourier transforms. Thisequivalence does not, however, mean that the use of the methods of FIGS.9A and 9B are equally easy to use, or that the DSPs which apply them usesimilar amounts of memory or operate with equal speed. In practice, therecovery function is usually more easily and quickly applied in thespatial frequency domain. Accordingly, the embodiment of FIG. 9A isregarded as the preferred embodiment of the method and apparatus of theinvention.

The magnitude of the improvement in image quality that is produced bythe method and apparatus of the invention is illustrated in FIGS. 11-14,each of which shows in dotted lines the calculated reflectivity, as afunction of distance X, for an exemplary object which takes the form ofa bar code test pattern, together with the corresponding calculatednormalized magnitude of the intermediate or final image signal, with andwithout the amplitude mask of the invention. FIG. 11, for example, showsthe reflectivity of the bar code test pattern together with theintermediate image signal that is produced when an imaging system of thetype shown in FIG. 6 uses the recovery function but not the amplitudemask of the invention. FIG. 12 shows the same test pattern together withthe final signal recovered from the intermediate image signal of FIG.11. Together, these figures show that even without an amplitude mask,the recovery function of the invention results in a final image signalwhich resembles the reflectivity of the test pattern from which it wasderived. FIG. 13 shows the calculated intermediate image signal that isproduced when an imaging system of the type shown in FIG. 6 uses boththe recovery function and the amplitude mask of the invention, and FIG.14 shows the calculated associated final image signal. Together, thesefigures show that when the recovery function and amplitude mask of theinvention are used in conjunction with one another, they cooperate andcomplement one another to produce a final image which represents asubstantial improvement over previously known imaging systems that usephase masks and phase mask-based approximations of recovery functions.

While the above-described embodiments make use of the discrete Fouriertransform pair and transform quantities back and forth between thespatial and spatial frequency domains, the present invention may bepracticed using other types of transform pairs, or other types ofconvolution or superposition integrals, together with the correspondingtypes of domains. The recovery function and recovery algorithm of theinvention might, for example, be implemented using wavelet transformpairs and transformations back and forth between the spatial frequencyand spatial domains. It will therefore be understood that the presentinvention is not limited to any particular type of transform pair, orequivalent convolution or superposition integral, and any particulartypes of transform domains.

New material of present application.

Referring to FIG. 15, a bar code laser scanner system 110 includes alaser 120 such as a gas laser or a solid state laser. An LED,superluminescent diode, or indeed any sufficiently small sufficientlymonochromatic illumination source, can be used. A broadband source witha filter to provide essentially monochromatic light can also be used ascan a pinhole or small aperture in front of an extended light source. Alight beam 125 from laser 120 passes through a spatial filter 130 and acondenser lens 132. Beam 125 then passes through a cubic phase mask(CPM) 134 and an optional amplitude mask 136. CPM 134 is preferably of atype shown in FIGS. 3A, 3B, 3D, 5A, and 5B. Beam 125 is then reflectedoff a rotating or vibrating mirror 138 into an operating depth of field(DOF) 140 which contains indicia 142 such as a bar code symbol. Unlikethe prior art, the illumination at the plane of indicia 142 is a complexlight intensity distribution rather than a single spot. The resultinglight intensity distribution, while scaling in size with the distancefrom lens 132, remains otherwise relatively invariant over the operatingDOF 140 for system 110.

A photodetector 144 viewing the operating DOF 140 through an optionaloptical system 143 receives light from the scanned indicia 142 thatrepresents the convolution of the scanning light intensity distributionwith the indicia reflectivity pattern and produces an intermediate imagesignal which is sent via a buffer 145 to a signal processor such as DSP146 which reconstitutes the image using digital signal processing. Theoutput of DSP 146 is converted to a bit serial representation of theindicia pattern in a digitizer 147, which is then decoded with a decoder148 and made available to a user via an interface 149. If DSP 146outputs a digital signal instead of an analog signal, digitizer 147 isnot needed.

Unlike the embodiments described with respect to FIGS. 1-14, we now havelocalized illumination with flood receive. The localized illuminationdistribution is similar to that shown in FIG. 7B and is scanned acrossthe indicia. The intermediate signal is the output of the photodiode andneeds to be processed by the DSP. This is the same intermediate signalas discussed with respect to the embodiments of FIGS. 1-14, but it iscreated in a totally different way. The signal from photodetector 144 isthe one-dimensional intermediate signal of FIGS. 9A-9B (element 9-1).Processing these signals accordance with FIGS. 9A-9B results in thedecoded final image signal.

Instead of a scanning mechanism in the optical path between theillumination optics and the target region, the illumination optics canbe translated or rotated so that the phase masked beam traverses acrossthe target region. Similarly, the illumination optics can be heldstationary and the indicia moved relative to the localized illuminationdistribution function. Also note that the misfocus error is dependent onthe distance between the illumination optics and the target region andis independent of the distance between the target region and thedetection optics.

Referring to FIG. 16A, a scanning system using this illuminationtechnique to obtain an enhanced DOF includes an illuminating source 150which generates a light beam which is transmitted to a phase mask 154through a lens or set of lenses 152. If the light beam has a desiredamplitude distribution, the beam goes directly from phase mask 154 to ascanning mechanism 158; otherwise the beam passes through an amplitudemask 156. Although the phase mask 154, amplitude mask 156, and lenssystem 152 are shown is a specific order, there may be systemconfigurations for which another arrangement of the same elements may bemore optimum. The light beam leaving scanning mechanism 158 illuminatesan object 160 and the reflected or scattered light from object 160passes through lens or lenses 162 to a photodetector 164. There may bealternate configurations that function adequately without lens system162. The word “reflected” in this specification and claims is used tomean either reflected, as from a specular surface, or scattered, as isthe case for all other surfaces. The signal produced by photodetector164 is processed by a DSP 166 which corrects for the known effects ofphase mask 154, thus resulting in an enhanced depth of field. Thismechanism may also be configured so that the return path to lens system162 is also scanned across object 160. In either case, this is theintermediate signal of FIGS. 9A & 9B (element 9-1). The subsequentsignal processing is described above in relation to FIGS. 9A-9B. Theappropriate recovery function is developed using the optical elementsdefined for the condenser optics instead of for the receive optics inaccordance with the process of FIG. 8.

The amplitude masking function is optionally accomplished by selectingor designing the light source so that it either partially or completelyhas the desired amplitude distribution, thereby by either completely orpartially eliminating the need for a discreet amplitude masking element.For instance, in systems designed referring to FIGS. 3A, 3B, 3D, 5A and5B, the amplitude masking function is optionally eliminated.

Referring to FIG. 16B, a detailed embodiment of the system of FIG. 16Ais shown, namely, a 1D hand held bar code reader 220. Reader 220includes an optics assembly 236 and a photosensor 234 for receiving theimage. The illumination system portion of reader 220 includes a driverand laser 256, a CPM 254, and an optics scanning assembly 252. An analogsignal processing circuit 232 and an analog to digital converter 234convert the analog output signal of image sensor 234 into anintermediate image signal that can be processed using digital methods inDSP 240. DSP 240 also includes a microcomputer 242, a read only memory(ROM) 244, a read write memory (RAM) 246, and a bus 248 forinterconnecting them. A suitable DMA controller 243 may also be includedin order to handle the storage of image data, thereby allowing computer242 to concentrate on the tasks of post-processing the digitalintermediate image signal to recover and decode the final image data.The operation of reader 220 is typically initiated either manually, bymeans of a suitable trigger T, or non-manually by means of anautotrigger arrangement.

The overall operation of reader 220 is controlled by a program stored inROM 244. Generally speaking, this program includes a reader controlportion, a decoding portion, and an image recovery portion. The readercontrol portion of the program deals with matters such as the startingand stopping of scanning, and the inputting and outputting of data viaan I/O interface 249. The decoding portion deals with matters such asidentifying the symbology used in the symbol being read and the decodingof that symbol in accordance with the rules that govern that symbology.Because programs for performing these functions are included incommercially available readers, such as the model numbers ST 3400 and IT4400 readers sold by the assignee of the present invention, they are notdiscussed in detail herein. The image recovery portion of the program isas discussed earlier in connection with FIGS. 9A and 9B.

Referring to FIG. 17, a specific example of a 1-D laser scanning opticalassembly that includes an amplitude mask and a phase mask that issuitable for practicing an embodiment of the present invention is shown.The components are shown in Table 2. This system utilizes two 2-D CPMelements placed back to back so as to create a 1-D CPM. The Gaussiancoefficient σ is that distance from the center at which the relativeoptical transmission is reduced to 1/e time its center value. A simplescanning wheel is used to achieve the scanning motion. The opticalelements are assembled in the order shown in FIG. 17.

TABLE 2 Element designation or description Dimension Description 420Unaphase 1101P HeNe laser λ = 632.8 nm distance 420 to 425 approx. 4inches (not critical) 425 f = 4.5 mm spatial filter objective lensdistance 425 to 426 <5 mm as adjusted properly 426 D = 10 micronsspatial filter pinhole diameter distance 426 to 436-1 393.5 mm 436-10.813 mm glass holder index n = 1.51509 436-2 0.6096 mm acrylicamplitude index n = 1.48978 mask amplitude mask Gaussian coefficient σ =0.0876 inch 436-3 0.183 mm glass holder index n = 1.51509 distance 436-3to 432 1.56 mm 432 lens 3.1 mm thick BK7 Biconvex radii of curvature =206.95 mm distance 432 to 434-1 1.62 mm 434-1 1.702 mm thick acrylic CPMindex n = 1.48978 C03 = 0.0787 (1/inch)² distance 434-1 to 434- 0.10 mm2 434-2 1.702 mm thick acrylic CPM index n = 1.48978 C03 = 0.0787(1/inch)² distance 434-2 to bar see appropriate text code test symbol442

Referring to FIG. 18, another embodiment uses a telocentric scanningprocess so that the scale of the intermediate image does not change overspace. A laser 310 sends a beam of light through a spatial filter 312 toa collimating lens 314. The collimated beam then passes through a CPM316, at which point it is still almost collimated. The beam then entersa telocentric scanner 320 which includes a scanning mirror 322 and alens or lenses 324. The scanned masked beam is reflected from an object350 and received by a detector 330. Detector 330 outputs a signal whichis processed as previously described to recover the image. The apparentscanning motion is shown by an arrow 340. The scan rate, in inches persecond, in the paper plane is essentially independent of the bar codelocation.

One can also use an array of lasers as opposed to a scanning laser. Thearray is electrically scanned, with the individual lasers or LED's beingactivated in a precise order. For example, see U.S. Pat. No. 5,258,605showing an electronically scanned array of light sources rather than asingle light source. If a 2-D image is being reconstructed, then a 2-Dphase mask and optional amplitude mask are required. The localizedillumination distribution function is similar to that shown in FIG. 7D.A 2-D recovery function is used to recover the final image. Otherwise,the 2-D system is essentially the same as the 1 -D system.

Referring to FIGS. 19A-19D, a series of scan profiles are presented madewith a conventional laser scanner constructed with the optical systemdescribed in Table 2 in which the cubic phase mask is removed. Note thatfor all of these figures the amplifier is configured so that an increasein signal level results in a signal displacement in a downwarddirection. The bar code test object consists of 5 narrow bars and 4narrow spaces having widths of 0.0049 inch followed by 3 large spacesand 3 large bars having widths of 0.050 inch. The figures show tracestaken with only a lens and Gaussian amplitude mask. The narrow barpattern appears as the group of four peaks within the first valley. Thegreater the amplitude of these four peaks, the more readable the patternis for a digitizer. FIG. 19A is taken at 410.0 mm, FIG. 19B is taken at402.0 mm, FIG. 19C is taken at 391.0 mm, and FIG. 19D is taken at 378.0mm. The distance noted is the distance between the lens and the testobject. Note that this system has a well defined signal profile over anoperating range slightly in excess of 19 mm.

Referring to FIGS. 20A-23B, scan profiles are shown for four differenttarget positions (351.5 mm, 409.0 mm, 420.0 mm, and 457.0 mm,respectively) over an operating range of approximately 106 mm. Theintermediate signal is shown in FIGS. 20A, 21A, 22A, and 23A, while thecorresponding reconstructed digital images are shown in FIGS. 20B, 21B,22B, and 23B. Note the significantly improved signal profiles whencompared to the same object viewed without the improved optical system.

Referring to FIG. 24, a method is outlined implementing the presentinvention. The intent of this method is to retrieve the informationencoded in an indicia, such as a bar code, that is located in a spacebeing scanned by a laser scanner. The bar code is located in a region ofspace in the field of view of the scanner and more specifically in aregion called the depth of field. Typically the depth of field togetherwith the field of view define an operational volume for the scanner. Itis the intent that the scanner will be able to retrieve the informationcontained in the indicia anywhere in this operational volume. Typicallythe indicia is illuminated with light generated by a laser diode asrepresented in step 24-1. The illumination is modified or predistortedin a known fashion using a phase mask that changes the phase of thelight locally as a function of position in the phase mask. It may alsobe necessary, in order to optimize the system operation, that theamplitude of the light (i.e. the intensity) by modified with anamplitude mask such that the amplitude changes in a known fashion as afunction of position in the amplitude mask. This modification of thephase and amplitude operationally occurs in step 24-2.

Regardless of the physical location of the elements used to modify thephase and amplitude, the light is focused with a lens or lenses suchthat the light forms a desired light intensity distribution patternthroughout the operational volume as shown in step 24-3. As representedwith step 24-4, the focused illumination is caused to scan across theindicia by having the light intensity distribution pattern move relativeto the indicia. Light scattered from the indicia is then received by aphotodetector in step 24-5. This may include the use of a lens or lensesand/or a mirror or mirrors to increase the optical efficiency of thiscollection process. The photodetector output, step 24-6, represents theintermediate image signal. This intermediate image signal is processedin accordance with either FIG. 9A or 9B as shown in step 24-7 toretrieve the information that was originally encoded in the indicia.

While the present invention has been described with reference to aparticular preferred embodiment and the accompanying drawings, it willbe understood by those skilled in the art that the invention is notlimited to the preferred embodiment and that various modifications andthe like could be made thereto without departing from the scope of theinvention as defined in the following claims.

What is claimed is:
 1. An imaging system for imaging an object locatedin a target region, comprising: an illumination source; an optical pathbetween said illumination source and said object; a first opticsassembly in said optical path; phase masking means in said optical pathfor receiving light and modifying a phase thereof as a function ofposition within said phase masking means, thereby creating phasemodified light having known effects therein; means for traversing saidphase modified light across said object in said target region; imagesensing means for receiving light reflected from said object andproducing an intermediate image signal therefrom; said intermediateimage signal including at least a misfocus error that is dependent upona distance between said first optics assembly and said object; andprocessing means for correcting for said known effects of said phasemodified light to produce a final image signal of said object.
 2. Animaging system according to claim 1, wherein said means for traversingincludes a scanning mechanism.
 3. An imaging system according to claim1, wherein said phase masking means is between said illumination sourceand said first optics assembly.
 4. An imaging system according to claim1, wherein said phase masking means is between said first opticsassembly and said target region.
 5. An imaging system according to claim1, further including amplitude masking means, in said optical pathbetween said phase masking means and said object, for varying anattenuated amplitude as a function of position within said amplitudemasking means, said function being substantially free ofdiscontinuities.
 6. An imaging system according to claim 5, wherein atleast two of said amplitude masking means, said first optics assembly,and said phase masking means comprise different parts of a singleelement.
 7. An imaging system according to claim 1, further including asecond optics assembly in a second optical path between said object andsaid image sensing means.
 8. An imaging system according to claim 1,wherein said illumination source is a laser light source.
 9. An imagingsystem according to claim 8, wherein said means for traversing is atelocentric scanning mechanism.
 10. An imaging system according to claim8, wherein said phase masking means is a cubic phase mask which isshaped so that it corrects approximately for said misfocus error.
 11. Animaging system according to claim 8, wherein said phase masking meansincludes at least one phase mask for receiving light incident thereonand modifying a phase thereof to cause an Optical Transfer Function(OTF) of said first optics assembly to be approximately invariant over apredetermined depth of field.
 12. An imaging system according to claim8, wherein said processing means includes a digital signal processor.13. An imaging system according to claim 8 in which said processingmeans is adapted to calculate a transform and inverse transform ofsignals generated during operation of said imaging system, and in whichsaid transform and inverse transform include at least a discrete fastFourier transform and an inverse discrete Fast Fourier transform.
 14. Animaging system according to claim 8, wherein said processing meansincludes a generalized recovery function including a finishing functionwhich is adapted to optimize performance of said imaging system for barcode reading applications.
 15. An imaging system according to claim 14,wherein said image sensing means is a photodetector, and in which saidgeneralized recovery function is a generalized 1 D recovery function.16. An imaging system according to claim 14, wherein said image sensingmeans is a photodetector, and in which said generalized recoveryfunction is a generalized 2D recovery function.
 17. An imaging systemaccording to claim 14, wherein said generalized recovery functionfurther includes a finishing function which takes into account at leastone of (a) an effect of non-uniform illumination across said targetregion, (b) aliasing effects, and (c) diffusion effects.
 18. An imagingsystem according to claim 1, wherein said illumination source is a lightemitting diode.
 19. An imaging system according to claim 18, furthercomprising amplitude masking means for receiving light from said phasemasking means and transmitting light to said means for traversing, saidamplitude masking means having an attenuated amplitude that varies as afunction of position within said amplitude masking means, said functionbeing substantially free of discontinuities.
 20. An imaging systemaccording to claim 19, wherein at least two of said amplitude maskingmeans, said first optics assembly, and said phase masking means comprisedifferent parts of a single element.
 21. An imaging system according toclaim 19, wherein said scanning mechanism is a telocentric scanningmechanism.
 22. An imaging system according to claim 19, wherein saidphase masking means is a cubic phase mask which is shaped so that itcorrects approximately only for said misfocus error.
 23. An imagingsystem according to claim 19, wherein said phase masking means includesat least one phase mask for receiving light incident thereon andmodifying a phase thereof to cause an Optical Transfer Function (OTF) ofsaid first optics assembly to be approximately invariant over apredetermined depth of field.
 24. An imaging system according to claim19, wherein said processing means includes a digital signal processor.25. An imaging system according to claim 19 in which said processingmeans is adapted to calculate a transform and inverse transform ofsignals generated during operation of said imaging system, and in whichsaid transform and inverse transform include at least a discrete fastFourier transform and an inverse discrete Fast Fourier transform.
 26. Animaging system according to claim 19, wherein said processing meansincludes a generalized recovery function including a finishing functionwhich is adapted to optimize performance of said imaging system for barcode reading applications.
 27. An imaging system according to claim 26,wherein said image sensing means is a photodetector, and in which saidgeneralized recovery function is a generalized 1D recovery function. 28.An imaging system according to claim 26, wherein said image sensingmeans is a photodetector, and in which said generalized recoveryfunction is a generalized 2D recovery function.
 29. An imaging systemaccording to claim 26, wherein said generalized recovery functionfurther includes a finishing function which takes into account at leastone of (a) an effect of non-uniform illumination across said targetregion, (b) aliasing effects, and (c) diffusion effects.
 30. An imagingsystem for imaging an object located in a target region, comprising: alaser illumination source; a first optical path between said laserillumination source and said object; a first optics assembly in saidfirst optical path; a cubic phase mask in said first optical pathbetween said first optics assembly and said object within 0.5 inches ofsaid first optics assembly for receiving light and modifying a phasethereof as a function of position within said cubic phase mask, therebycreating phase modified light having known effects therein; an amplitudemask in said first optical path; means for moving said phase modifiedlight across said object; a photodetector for receiving light reflectedfrom said object and producing an intermediate image signal therefrom;said intermediate image signal including at least a misfocus error thatis dependent upon a distance between said first optics assembly and saidobject; a second optics assembly in a second optical path between saidobject and said photodetector; and processing means for correcting forsaid known effects of said phase modified light to produce a final imagesignal of said object.
 31. A method of scanning for a laser scanningsystem suited for reading indicia located in a target region, comprisingsteps of: generating an illumination beam for illuminating an indicialocated in an operational depth of field; changing locally a phase ofsaid illumination beam as a function of position before said beamilluminates said indicia; changing locally an amplitude of saidillumination beam as a function of position before said beam illuminatessaid indicia; receiving light reflected from said indicia; convertingthe received light to an intermediate image signal; and processing saidintermediate image signal such that said operational depth of field isextended.
 32. A method of modifying a laser beam to maximize the systemresolving ability as a bar code indicia is moved throughout anoperational depth of field, comprising steps of: generating a laserillumination beam; modifying a phase of said illumination beam afunction of position in said beam; modifying an amplitude of saidillumination beam as a function of position in said beam; scanning saidphase and amplitude-modified beam across said bar code indicia;receiving light reflected from said bar code indicia; converting thereceived light to an intermediate image signal; and processing saidintermediate image signal using a precalculated recovery function toreduce effects of a position of said bar code indicia.
 33. A method ofscanning a barcode indicia, said indicia containing information encodedtherein, comprising steps of: illuminating said indicia in an operatingrange with light; modifying an amplitude and phase of said light toreduce variations in a localized illumination distribution of said lightover said operating range before said light illuminates said indicia;directing said light to said operating range; scanning said light acrosssaid indicia; receiving light scattered from said indicia; convertingsaid scattered light to an intermediate image signal; and processingsaid intermediate image signal to recover said information encoded insaid indicia.
 34. A method of increasing an operating depth of field,comprising: providing illumination; distorting a phase of saidillumination in a predetermined manner at a specific distance from anobject; directing said distorted illumination to said object; receivinglight scattered from said object; converting said received light to anintermediate image signal; and processing said intermediate image signalusing a precalculated recovery function to reduce effects of saidspecific distance.